Improving graphics display technologies provide for highly realistic rendering of images in simulated environments such as games. Even modestly priced graphics adapters and gaming systems can render remarkably realistic scenes.
Notwithstanding the high capability of graphics hardware, realistic rendering of outdoor settings continues to present a challenge. For example, in rendering a daylight setting involving a number of plants, a system must represent how sunlight and shadow contribute to the appearance of the plants. Realistically representing leaves is particularly difficult because of the complex underlying structures of leaves that affect how leaves reflect and transmit light. Leaves are partially translucent as a function of their physical and biochemical structures. The degree to which a leaf reflects, scatters, or transmits light not only varies from plant to plant, but varies across the surface of the leaf because of variations in thickness and other characteristics.
Many efforts to render leaves rely on using bidirectional reflectance distribution functions (BRDFs) and bidirectional transmittance distribution functions (BTDF). BRDFs and BTDFs capture the translucence of leaves by modeling the reflectance and transmission of light for individual leaves as a two-dimensional function of each point of a leaf relative to that point's position on the leaf surface. Some BRDF/BTDF models have been devised based on geometrical and biochemical analyses of leaf samples. Developing these models requires detailed knowledge of internal leaf structures, and the resulting models frequently yield large data files.
To avoid this level of complexity, more compact models have been generated based on experimental data related to inorganic materials. These models are easier to create and require less data storage, but fail to accurately represent the subsurface scattering that allows for realistic rendering.
Even if a leaf can be rendered realistically, realistically representing how a leaf is illuminated by direct sunlight, reflections, shadows, and other environmental light represents an entirely different problem. Because of the way that leaves are bunched together, accurate depiction of how leaves are illuminated by sunlight involves detailed ray tracing. Ray tracing generally does not support the fast run-time shading calculations desired to render leaves in real time.
Other lighting calculation solutions, such as precomputed radiance transfer (PRT) can calculate illumination more quickly than ray tracing. However, PRT is much better suited to modeling low energy, low frequency lights, such as manmade indoor lighting, than it is for high energy, high frequency lighting such as sunlight. Using PRT to model sunlight tends to do a poor job of representing shadows and other illumination details resulting from high-frequency illumination.